Transcript AGVela.pptx

The role of orbiting resonances in the vibrational relaxation of
I2(B,v’=21) by collisions with He at very low energies: A
theoretical and experimental study
A. García-Vela1, Iván Cabanillas-Vidosa2, J.C. Ferrero2, and G.A. Pino2
Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas,
C/ Serrano 123, 28006 Madrid, Spain
2 Centro Láser de Ciencias Moleculares, INFIQC, Departamento de Fisicoquímica, Facultad de Ciencias
Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 500 Córdoba, Argentina
1
Introduction: In the last years there has been a controversy about whether the origin of the unexpectedly large cross sections found
experimentally for the I2(B,v’) vibrational relaxation induced by collisions with He at very low collision energies was related to the presence of
orbiting resonances [1-3]. More recently, measured cross sections for I2(B,v’=21) vibrational relaxation upon low temperature collisions with He
exhibited for the first time clear peaks at given collision energies (Fig. 1) that were attributed to orbiting resonances of the I2(B,v’=21)-He vdW
complex formed in the low energy collisions [4,5]. Further recent wave packet simulations (assuming zero total angular momentum) confirmed that
the peaks in the experimental cross sections are the signature of orbiting resonances of the I2(B,v’=21)-He complex [6].
Calculated total and inelastic partial cross
sections for the I2(B,v’=21) + He collisions
vs energy
Total (elastic plus inelastic) cross sections
are obtained for different initial rotational
states j’ of I2(B,v’=21,j’) (left figure). In the
middle figure partial inelastic cross
sections for the channel I2(B,v’=21,j’) + He
 I2(B,v’’=21,j’’) + He (due to tunneling)
obtained for several initial j’ states are
shown.
Averaged total and inelastic partial cross
sections are obtained by summing the
j’=0-9 contributions weighted with a
Maxwell-Boltzmann distribution
corresponding to a temperature T=0.5 K
(right figures). The averaged cross sections
present a series of peaks which
correspond to the positions of the orbiting
resonances of the I2(B,v’=21)-He complex.
The positions of three of the theoretical
peaks coincide very nicely with those of the
experimental peaks (Fig. 1).
Fig. 1. Experimental
cross sections [1].
Averaged inelastic partial cross section corresponding to the Δv’=0
channel, obtained by weighting each j’ contribution to the cross section
with an equiprobable distribution assigning a weight 1/10 to each j’
contribution. The positions of the peaks are essentially the same as
those of the peaks of the cross sections averaged with the MaxwellBoltzmann distribution, indicating that the peak positions found
theoretically are independent on the weighting distribution used to
average the cross section, and that these peaks actually reflect the
positions of the I2(B,v’=21)-He orbiting resonances.
Averaged inelastic partial cross sections corresponding to the vibrational
relaxation channels I2(B,v’=21) + He  I2(B,v’’=v’-1, v’-2, v’-3) + He (the
Δv’=-1, -2, and -3 channels) using the Maxwell-Boltzmann distribution.
The cross sections exhibit two peaks at 0.11 and 0.39 cm-1, and a broader
bump around 2 cm-1. The vibrational relaxation process being faster than
the tunneling process associated with the Δv’=0 channel would produce
broader peaks that would give rise to the less resolved structure of these
cross sections. The absence of J>0 contributions in the calculation could
also be responsible of the less resolved structure of peaks.
Experimental rate constants and cross sections for the Δv’=0 and Δv’<0
vibrational relaxation channels [6]. In these new experimental cross
sections a new peak (albeit weak) at around 2.7 cm-1 is found. It is noted
that this peak position agrees very well with a theoretical peak found at
2.71 cm-1 for the Δv’=0 channel. Another point of agreement between
experiment and theory is that the structure of peaks is more
pronounced for the Δv’=0 channel than for the Δv’<0 vibrational
relaxation channels.
Conclusions: The cross sections calculated for the low energy collisions of I2(B,v’=21) with He exhibit a pronounced structure of peaks
originated by orbiting resonances of the I2(B,v’=21)-He van der Waals complex formed upon the collisions. This structure of peaks is similar to that
found in the experimental cross sections. Actually, out of the five peaks found in the measured cross sections, the first four peaks (at 0.82, 1.17,
1.67, and 2.7 cm-1) have nearly coincident positions with those of four of the theoretical peaks. This result confirms that the peaks of the
experimental rate constants and cross sections are originated by orbiting resonances of the I2(B,v’=21)-He complex, and the role played by these
resonances in enhancing the I2 vibrational relaxation.
References
[1] J. Tusa, M. Sulkes, and S.A. Rice, J. Chem. Phys. 70, 3136 (1979).
[2] C. Cerjan and S.A. Rice, J. Chem. Phys. 78, 4952 (1983).
[3] W.R. Gentry, J. Chem. Phys. 81, 5737 (1984).
[4] I. Cabanillas-Vidosa, G.A. Pino, C.A. Rinaldi, and J.C. Ferrero, Chem. Phys. Lett. 429, 27 (2006).
[5] I. Cabanillas-Vidosa, C.A. Rinaldi, G.A. Pino, and J.C. Ferrero, J. Chem. Phys. 129, 144303 (2008).
[6] A. García-Vela, I. Cabanillas-Vidosa, J.C. Ferrero, and G.A. Pino, Phys. Chem. Chem. Phys. 14, 5570 (2012).
Acknowledgements: A. G.-V. was funded by CICyT, Ministerio de Ciencia e innovación (MCINN), Spain, Grant No. FIS-2011-29596-C02-01, the
Consolider program, MCINN, Spain, Grant No. CSD2007-00013, COST Action program, Grant No. CM1002, and the Centro de
Supercomputación de Galicia (CESGA).The experimental work was supported by CONICET, FonCyT, SeCyT, and MinCyT Córdoba.